1. Field of the Invention
The present invention relates generally to the field of antennas and more particularly to an extendable antenna for use with portable radio communication devices.
2. Description of the Prior Art
Extendable antennas are widely used for receiving and transmitting radio frequency signals in portable radio communication devices such as cordless telephones and cellular/PCS (Personal Communication Service) telephones. Generally, such extendable antennas operate in an extended position when the telephone is in the "talking" or "transmit and receive" mode, but must remain functional in a retracted position to allow the telephone to receive an incoming call while in the "stand-by" or "receive only" mode. The extendable antennas are typically implemented in a dual-antenna configuration, as shown in FIG. 1A and 1B, where a quarter-wavelength helical antenna element 12 and a quarter-wavelength whip antenna element 14 are separated from each other at an interval such that the whip antenna element 14 is operational when the antenna 10 is in the extended position while the helical antenna element 12 is operational when the antenna 10 is in the retracted position. The whip antenna element 14 and the helical antenna element 12 do not affect performance and operate independent of each other, with the whip antenna element 14 being active during the transmit and receive mode of operation and the helical antenna element 12 being operative for receiving the incoming signals during the stand-by mode of operation.
However, since such conventional antennas are operated in the retracted position only by an inherently less efficient quarter-wavelength helical antenna, they generally suffer from narrow operating bandwidth and low radiation efficiency in the retracted or stand-by mode of operation. Accordingly, the present invention aims to provide a capacitive coupled extendable antenna which is capable of operating with wider operating bandwidth and improved radiation efficiency while achieving more stabilized operating characteristics and mechanical reliability.